Immune cell interactions that lead to loss of tolerance and lupus pathology are complex. T cells are critical for helping B cells, and they can also be directly pathogenic. B cells were originally thought to mainly be important for autoAb secretion, but as our work and that of many others has indicated, B cells play Ab- independent roles. These discoveries have focused attention on Ag presentation in SUE. However, the precise functions of B cells, aside from autoAb secretion, have not been demonstrated;B cells determine lymphoid architecture and secrete cytokines as well. DCs are also potent APCs and whether they are required to initiate autoimmunity, or to maintain it, have not been investigated. The plasmacytoid DC (pDC) is particularly interesting in this regard as it secretes large amounts of cytokines, especially type I IFN, in response to TLR signals, but also appears capable of Ag presentation. The clinical relevance of these questions is intensified by the emerging data indicating that B cell depletion is effective therapy in SLE and RA patients. What is the mechanism? Why do only some patients respond? Are DCs additional synergistic targets? We propose to extend our lab's longstanding interest in these questions by developing clinically relevant animal models that are both treatment-based and also genetic. To do this, we have developed and propose to extend some very exciting and novel genetic tools using Tg mice made with modified bacterial artificial chromosomes (BACs). BACs contain approximately 150kb of genomic DNA that typically include all the necessary genetic control elements to mediate tissue-specific expression of genes. We will use particular BACs that contain B cell- (hCD20), pDC- (BDCA-2) or pan-DC- (DC-LAMP) specific genes to express either Cre (allowing tissue-specific gene deletion of "floxed" alleles) or diphtheria toxin A (which ablates the target cell type). These mice are in various stages of construction and crossing to lupus-prone MRL/lpr mice. Our Aims are: 1) To examine the role of B cells in murine lupus using in vivo depletion of B cells with anti-CD20 Abs and by B cell-specific and inducible deletion of MHCII and cytokine genes;2) To investigate why lupus- prone mice are more resistant to in vivo depletion of B cells with anti-CD20, as this could be highly relevant to clinical application;and 3) To use genetic approaches to investigate the roles of pDC and all DCs in antigen presentation and other pro-inflammatory functions, again by cell ablation and tissue-specific deletion of MHCII. These studies should give us much better insight into the mechanisms and problems of B cell depletion during ongoing disease and should shed new light on the roles of both pDCs and conventional DCs in promoting murine lupus.